1. Field of the Invention
The present invention relates to a semiconductor device and a method for producing the same, more particularly relates to a semiconductor device having a conductive layer in which a work function of a conductive material located at a boundary with an insulating film formed on a substrate is controlled to near the substantial center of an energy band gap of a substrate material, that is, the xe2x80x9cmid-gapxe2x80x9d, and to a method for producing the same.
2. Description of the Related Art
In semiconductor devices in recent years, complete separation among elements has become easy by using a silicon-on-insulator or semiconductor-on-insulator (SOI) substrate as the substrate. Further, it is known that, if such an SOI substrate is used, the control of the latchup and the software error peculiar to a complementary metal oxide semiconductor (MOS) transistor (CMOSTr) becomes possible, so studies have been conducted on the increase of speed and increase of reliability of large-scale integrated circuits (LSIs) comprised of CMOSTrs using SOI substrates having silicon (Si) active layers of a thickness of about 500 nm from a relatively early stage.
Further, recently, it has been learned that if the Si active layer of the SOI substrate surface is made further thinner to about 100 nm and the impurity concentration of a channel region is controlled to be relatively low to make substantially the entire Si active layer depleted (make it a full depletion type), excellent characteristics such as suppression of a short channel effect and improvement of a current driving capability of the MOSTr are obtained.
On the other hand, as a gate electrode material, polycrystalline silicon doped with an n-type impurity (n+poly-Si) has been frequently used in the past. However, in order to set a threshold voltage (Vth) of an n-channel MOS transistor (NMOSTr) to near 0.5 to 1.0V of a usual enhancement type MOS transistor by using n+poly-Si for the gate electrode material, it is necessary to control the impurity concentration of the channel region to about 1017/cm3 or more.
Further, in order to prepare a full depletion type enhancement type MOSTr, the method has been studied of using a polycrystalline silicon doped with boron as a p-type impurity (p+poly-Si) as the gate electrode material in place of the n+poly-Si for the gate electrodes of the NMOSTr.
In this method of using p+poly-Si for the gate electrodes of an NMOSTr, if the impurity is not included in the channel region (non-doped), Vth becomes substantially 1.0V. Further, where it is intended to make Vth a further lower value, it has been necessary to perform counter-doping to dope an n-type impurity, for example, phosphorus (P+), in the channel region of the NMOSTr. However, when performing the counter-doping, the short channel effect is increased, so this is not preferred for a miniaturized LSI.
In this way, in any case of using n+poly-Si and p+poly-Si as the gate electrode material, in the preparation of a semiconductor device using an SOI substrate having a fine structure with a thin silicon active layer, it was extremely difficult to control the Vth of a full depletion type MOSTr to a suitable value of about 0.5V.
Further, even in the case of preparing a MOSTr with a channel region of a partial depletion type, careless increase of the impurity concentration of the channel region is not preferred in that it increases the drain leak current.
Further, semiconductor devices using bulk silicon substrates have been being miniatured as well. When using a bulk silicon substrate, it is not possible to form a surface channel type MOSTr resistant to a short channel effect simultaneously in both of the N-channel and the P-channel using only n+poly-Si for the gate electrodes. Therefore, as shown in FIGS. 1A to 1C, a so-called dual gate process of using n+poly-Si for the NMOSTrs (shown in FIG. 1A) and using p+poly-Si for the p-channel MOS transistors (PMOSTr) (shown in FIG. 1B) has been studied for the purpose of adjustment of the Vth by using the work function of the gate electrodes.
However, in this dual gate process as well, when using poly-Si gate electrodes 14a and 14b of different types of dopants between the NMOSTr (shown in FIG. 1A) and the PMOSTr (shown in FIG. 1B), as shown in FIG. 1C, there is a problem that impurities in the gate electrodes diffuse into each other (indicated by arrows in the figure) in parts at which the n+poly-Si gates of the NMOSTr and the p+poly-Si gates of the PMOSTr are connected and that the work functions of the gate electrodes largely fluctuate.
This problem becomes particularly conspicuous when a silicide is further formed at an upper layer of the poly-Si to be made to tungsten polycide (W-polycide) in order to lower the resistance of the gate electrodes formed by the dual gate process, as shown in FIGS. 1A to 1C, since the diffusion coefficient of the dopant in the tungsten silicide (WSix) is extremely large.
Note that, in FIGS. 1A to 1C, 11 denotes a silicon substrate, 12 a field oxide film, 13 a gate insulating film, 14a a gate electrode of a NMOS transistor, 14b a gate electrode of a PMOS transistor, 14c a junction portion of the gate electrodes of the NMOS transistor side and the PMOS transistor side, and 15 an inter-layer insulating film.
In this way, even in a case where a SOI substrate is used and even in a case where a bulk silicon substrate is used, in order to deal with the miniaturization of semiconductor devices in the future, there is a problem with usage of different types of poly-Si for the gate electrode material. In place of this, it has been considered necessary to use a gate electrode material having a work function near the mid-gap.
The energy band of the semiconductor has a structure where an electronically filled band (a filled band or a valence band) and an empty band (conduction band) are separated by a prohibit band, and in the present invention, a gate electrode material having a work function near the mid-gap means a conductive material which has a work function (energy difference between a vacuum level and a Fermi level) almost the same as that near the center (near mid-gap) of the width of this prohibit band (band gap).
Summarizing the problem to be solved by the invention, among the gate electrode materials having a work function near this mid-gap, refractory metal silicide or refractory metal does not directly react with the SiO2 and does not cause conspicuous deterioration of the gate withstand voltage, so attracts attention as particularly preferred material and has been studied as gate electrode material.
However, as shown in FIG. 2, when a gate insulating film 23 is formed on a silicon substrate 21 and a gate electrode is further formed on this by a single layer film 24 made of WSix or another refractory metal silicide, there is the problem that a reduction of the gate insulation withstand voltage or a reduction of a gate capacity occurs in comparison with the case of the related art where a gate electrode such as poly-Si (or W-polycide) is used. Note that, in FIG. 2, 21 denotes a silicon substrate, 22 a field oxide film, 23 a gate insulating film, 24 a gate electrode made of a single WSix layer, and 25 an inter-layer insulating film.
The reduction of the gate insulation withstand voltage is not preferred for a next generation device which is further miniaturized and where the gate oxide film is made further thinner. Further, the reduction of the gate capacity invites a reduction of the drive capability of the transistors etc. and as a result ends up lowering the operating speed of the device.
An object of the present invention is to provide a semiconductor device having a conductive layer, preferably a gate electrode, using a conductive material having a work function near the mid-gap of the energy band gap of the substrate material, preferably silicon, at least in the vicinity of the boundary with an insulating film, preferably gate oxide film, formed on the substrate, not causing deterioration of the insulation withstand voltage of the insulating film formed on the substrate, not causing a reduction of the capacity (gate capacity) of the conductive layer after formation, and maintaining the operating speed of the device and a method for producing the semiconductor device.
The present inventor discovered the fact that the reduction of the gate insulation withstand voltage and the reduction of the gate capacity in the case of forming the gate electrodes by a single layer film made of WSix or another refractory metal silicide were due to the heat treatment step performed after the gate electrodes were formed and to the As and other impurities being taken into the WSix or other refractory metal silicide and resulting growth of the grain of the WSix or other refractory metal silicide.
Accordingly, if the growth of the grain of the WSix or other refractory metal silicide can be suppressed by a certain method, it can be expected that a gate electrode can be obtained in which the short channel effect is suppressed and the operating speed of the device is maintained without adding any change to conditions of the heat treatment step or step for doping the impurity and without inviting a reduction of the gate insulation withstand voltage or a reduction of the gate capacity.
The present inventor engaged in intensive studies and as a result discovered that by using refractory metal silicide, refractory metal or other conductive materials having a work function near the mid-gap of the energy band gap of the substrate material (silicon) and causing doping of a certain type of impurity into the conductive material, it is possible to suppress the grain growth of the conductive material and thereby completed the present invention.
Namely, the present invention provides a semiconductor device comprising a substrate, an insulating film formed in the substrate, a conductive layer formed on the insulating film and having at least a part in contact with the insulating film made of a conductive material having a work function near a substantial center of an energy band gap of the substrate material, and a takeout electrode formed in the substrate, characterized in that the conductive material contains a predetermined amount of impurity.
In the semiconductor device of the first aspect of the invention, the impurity is preferably an impurity suppressing the grain growth of the conductive material. As the impurity, more concretely, oxygen, nitrogen, boron, etc. is more preferably used.
Alternatively, the conductive layer preferably has a conductive material containing impurity having different concentrations in a depth direction (perpendicular direction with respect to the substrate) and having a work function near the substantial center of the energy band gap of the substrate material, for example, a refractory metal silicide layer or a refractory metal layer, more preferably has a conductive material with a center region with respect to a depth direction thereof containing an impurity having a higher concentration than those of upper and lower regions thereof having the work function near the substantial center of the energy band gap of a substrate material, for example, a refractory metal silicide layer or a refractory metal layer.
Alternatively, more preferably, the conductive layer contains two or more types of impurities. Preferably, at least one of the two or more types of impurities is oxygen, nitrogen, or boron. Each of the two or more types of impurities further preferably are contained in a concentration of 1xc3x971019/cm3 to 1xc3x971021/cm3.
Further, the substrate material is preferably silicon, and the conductive material is preferably a refractory metal silicide or a refractory metal.
As the refractory metal silicide, one, two or more types selected from a group consisting of tungsten silicide (WSix), molybdenum silicide (MoSix), tantalum silicide (TaSix), and titanium silicide (TiSix) can be exemplified.
As the refractory metal, one, two or more types selected from a group consisting of tungsten (W), tantalum (Ta), and titanium (Ti) can be exemplified.
Further, as the takeout electrode, for example, a source and a drain formed on the substrate can be mentioned.
The present invention provides, second, a semiconductor device comprising a silicon substrate, a gate insulating film formed in the silicon substrate, a gate electrode formed on the gate insulating film and having at least a part in contact with the gate insulating film made of a refractory metal silicide layer containing an impurity or a refractory metal layer containing an impurity, and a takeout electrode formed in the silicon substrate. The second aspect of the invention more concretely specifies the semiconductor device of the first aspect of the invention.
In the second aspect of the invention, as the silicon substrate, an n-type silicon semiconductor substrate, a p-type silicon semiconductor substrate, an SOI substrate, etc. can be used.
The present invention provides, third, a method for producing a semiconductor device comprising steps of forming an insulating film in a substrate, forming a conductive layer made of a conductive material having a work function near a substantive center of an energy band gap of the substrate material on the insulating film, doping an impurity into the conductive layer, and forming a takeout electrode in the substrate.
In the third aspect of the invention, the step of doping the impurity into the conductive layer preferably has a step of doping the impurity into the conductive layer by an ion implantation process, a step of doping a impurity suppressing a grain growth of the conductive layer into the conductive layer by the ion implantation process and/or a step of forming the conductive film containing the impurity on the insulating film by a chemical vapor deposition process (CVD process).
Further, the step of doping the impurity into the conductive layer preferably has a step of doping the impurity so that an impurity concentration varies in a depth direction and more preferably has a step of doping the impurity into the conductive layer so that a concentration of the impurity contained in a center region thereof with respect to a depth direction thereof becomes higher than impurity concentrations of upper and lower regions thereof.
More concretely, the step of doping the impurity into the conductive layer preferably has a step of doping oxygen, nitrogen, or boron into the conductive layer.
Further, the step for doping the impurity into the conductive layer preferably has a step of doping two or more types of impurities into the conductive layer and, in this case, more preferably, has a step of doping at least oxygen, nitrogen, or boron into the conductive layer and further preferably has a step of doping each of the two or more types of impurities into the conductive layer with the concentration of 1xc3x971019/cm3 to 1xc3x971021/cm3.
In the third aspect of the invention, as the substrate, preferably a silicon substrate such as a p-type silicon semiconductor substrate, n-type silicon semiconductor substrate, or SOI substrate is used.
The step of forming the conductive layer made of the conductive material having the work function near the substantive center of the energy band gap of the substrate material preferably has a step of forming a refractory metal silicide layer or a refractory metal layer on the substrate.
The step of forming the refractory metal silicide layer preferably has a step of forming a layer made of one, two or more types selected from a group consisting of tungsten silicide (WSix), molybdenum silicide (MoSix), tantalum silicide (TaSix), and titanium silicide (TiSix).
Further, the step of forming the refractory metal layer preferably has a step of forming a layer made of one, two or more types selected from a group consisting of tungsten (W), tantalum (Ta), and titanium (Ti).
Further, the present invention provides, fourth, a method for producing a semiconductor device comprising steps of forming a gate insulating film in a silicon substrate, forming a conductive layer made of a conductive material having a work function of a substantial center of an energy band gap of the silicon on the gate insulating film, doping an impurity into the conductive layer, forming a gate electrode by processing the conductive layer, and forming a takeout electrode in the silicon substrate. The fourth aspect of the invention more concretely specifies the invention of the method of production of the third aspect of the invention and is the method for producing a semiconductor device according to the second aspect of the invention.
In the fourth aspect of the invention, as the silicon substrate, a p-type silicon semiconductor substrate, n-type silicon semiconductor substrate, SOI substrate, etc. can be preferably used.
The semiconductor devices of the first and second aspects of the invention are characterized in that at least a part of a conductive layer, preferably gate electrode, in contact with the insulating film, preferably gate insulating film, is made of a conductive material having a work function near the substantive center of the energy band gap of the substrate material, preferably refractory metal suicide or refractory metal, and the conductive material contains an impurity.
Accordingly, the semiconductor devices of the first and second aspects of the invention are semiconductor devices having conductive layers (gate electrodes) in which a so-called short channel effect is suppressed and the operating speed of the device is maintained. In addition, they are semiconductor devices in which the dielectric breakdown of the insulating film of the lower layer accompanied with grain growth of the refractory metal silicide or the refractory metal or other conductive material or in a MOSTr the insulating film (gate insulating film) dielectric breakdown and the reduction of the gate capacity, which had become a problem of the related art, are suppressed.
Further, when the conductive material of the conductive layer of the semiconductor device of the present invention contains two or more types of impurities, in comparison with a case where one type of impurity is doped, it is possible to more effectively suppress the grain growth of the conductive layer. Accordingly, even in a case where a thinner insulating film, for example a gate insulating film having a thickness of about 4 nm, is formed, the semiconductor devices having a conductive layer, that is, gate electrodes, excellent in a reliability without causing deterioration of the insulation withstand voltage are provided.
Further, according to the method of production of the semiconductor devices of the third and fourth aspects of the invention, a semiconductor device can be produced having a conductive layer, that is, gate electrodes, in which the short channel effect is suppressed and the operating speed of the device is maintained without adding any change to the conditions of the heat treatment step and the step of doping the impurity after this and without causing a reduction of the insulation withstand voltage and the reduction of the gate capacity.
Further, by doping the impurity nonuniformly in the depth direction of the conductive layer, preferably so that the center portion with respect to the depth direction of the conductive layer has a relatively high concentration and the upper and lower regions thereof have a relatively low concentration, a MOSTr having insulating film boundary surface characteristics similar to those of the case where the impurity is not doped can be formed. Accordingly, according to the methods of production of a semiconductor device of the present invention, the degree of freedom of process design of the semiconductor device is not lowered.
Further, when there is the step of doping the impurity into the conductive layer by the ion implantation process, the impurity can be ion implanted with a correctly controlled acceleration energy and dosage.
Accordingly, according to the present invention, the degree of integration of the LSI can be improved, the drive capability of the MOSTr can be improved according to the design rule, and high speed operation of the device becomes possible.